416 CHEMICAL ENGINEERING
In general, it may be said that in all cases where the application of stem corrections is neglected, which includes a.vast majority of ordinary routine laboratory temperature measurements, more accurate temperature measurements would be attained by the use of thermometers graduated as partial immersion thermometers. The same statement would apply for measurements at the higher temperatures (above 200°C. or thereabouts), even if stem corrections are applied, when the ordinary method of estimating average stem temperature is used instead of the more accurate capillary thermometer method. At the lower temperatures, on the other hand, a slight advantage rests with the total immersion thermometer, if the stem correction is determined and applied in the usual manner, i.e., by the intelligent use of an auxiliary thermometer to determine the average stem temperature. Thermometers graduated in intervals smaller than-0.5°C. should not, in general, be graduated as partial immersion thermometers, if the accuracy of which they are capable is desired, unless such finer graduation be deemed of sufficient importance solely from the standpoint of convenience in reading.
Other Sources of Error in High-temperature Mercurial Thermometers.— Aside from the error that may be introduced in the use of high temperature mercurial thermometers by a failure to observe the proper condition of immersion, there are two common sources of error that can be attributed to actual defects in the manufacture of the instrument. These are first, insufficient pressure above the mercury to prevent a breaking of the mercury column, or evaporation at the surface of the mercury, and secondly, improper or insufficient annealing for use at high temperatures. This latter defect may result in a rise of the reading with continued 'heating amounting to as much as 20°C. in extreme cases.
In discussing the question of filling under pressure it may be noted that mercury boils at approximately 357°C., and evaporates fairly rapidly at much lower temperatures. Experiments have shown that even for use above 100°C. the filling under pressure of that part of the capillary above the column with an inert gas such as nitrogen is desirable. The amount of pressure that must exist above the surface of the mercury to prevent evaporation or breaking of the mercury column varies with the temperature and the construction of the upper portion of the stem. This pressure may be anywhere from 2 to 20 atmospheres.
The failure to fill high-temperature thermometers under the proper pressure is often the cause of large errors. This defect can be detected sometimes by the broken appearance of the mercury column, and by drops of mercury that have distilled from the top, but often the column breaks in a part of the stem not visible, and the defect is not detected.
Improper annealing can be detected only by the checking of the indications of the instruments from time to time. Laboratories usually have facilities for checking the reading either in melting ice or in steam or perhaps comparing the thermometers from time to time with a thermometer known to give reliable readings. Industrial thermometers should also be tested for this source of error, although annealing changes due to heating are not liable to be appreciable below 300°C. for this type.
Indicating and Recording Thermometers.—The term "indicating" is usually employed to designate thermometers of the dial-and-pointer type, rather than those constructed of mercury and glass. Indicating thermometers may or may not be distance-reading, i.e., so constructed as to allow the indicator to be placed at some distance from the bulb. A distance-reading indicating thermometer is